Existing active denial systems involve the use of millimeter-waves, directed onto the subject using a focusing system such as a focusing reflector, lens, flat-panel array antenna, or phased-array system. The properties of these existing focusing systems can be described in terms of a traditional rectangular Cartesian coordinate system, with x, y, and z axes. Where the direction of propagation of a beam is centered along the z-axis, traditional focusing systems cause the beam to converge or diverge approximately equally in both x and y directions. If the beam is converging as it leaves the aperture of the device, it will come to a focus—a plane of minimum extent in x and y—at some particular location along the z-axis. As the beam propagates beyond this point, the beam will diverge.
Generally, over the distances over which these devices are effective, atmospheric absorption of millimeter waves is small, so the average power density in the beam at any location along the z-direction is given by the total power emitted by the device divided by the effective area of the beam (since the beam intensity will not simply drop to zero at some distance in x or y away from the z-axis, the “boundary” of the beam is usually defined, for example, as the contour at which the intensity of the beam falls to 1/e2 of its peak intensity along the z-axis). In the case in which the beam is converging as it leaves the device aperture, the beam will have a plane of maximum intensity (at the plane of minimum beam area) with decreasing intensity at locations in the z-direction that are either further away from or nearer to the device than the plane of maximum intensity.
One issue with the variation of intensity with distance along the beam is that there is a range of intensity or power density that is useful in the active denial application. There is a minimum power density below which the subject is not adequately deterred, and a maximum power density above which the beam can cause damage to tissue. Generally, it is preferable that no portion of the beam have an intensity exceeding the damage threshold. The beam will always have a maximum distance beyond which the intensity falls below the effectiveness threshold, but in some configurations in which the beam is converging along both the x and y axes as it leaves the aperture of the apparatus that generates and emits the beam, there will also be a minimum distance from the apparatus within which the beam intensity falls below the effectiveness threshold. Therefore, one must consider the beam intensity with regard to distance from the device for uses such as crowd control or close-range situations.
The distance over which a traditionally focused electromagnetic beam can remain effectively collimated (i.e., not significantly converging nor diverging) is related to the wavelength and the effective diameter of the beam. FIG. 1(a-d) show beam diameters and power densities as a function of distance of propagation away from the device for several prior art devices having “circular” focusing elements (i.e., that generate beams that depend only upon distance along the z-axis and radial distance away from the z-axis, but not upon angle around planes parallel to the x-y plane). FIGS. 1 (a) and (b) show the evolution of beam diameter and power density for devices having 1 meter diameter apertures, one focused so as to create a maximum beam intensity at a distance of 100 meters from the device and the other configured to be collimated at the plane of the aperture. For simplicity of comparison, each beam intensity curve is shown normalized to a peak power density of 1 W/cm2. The associated total power requirements to transmit the beams shown are 3.9 kW (per W/cm2) for the collimated beam, and 675 W (per W/cm2) for the focused beam. Using a focused beam allows a greater than five-fold reduction in required peak power, but with these focal conditions the focused device will likely be ineffective for distances substantially less than 50 meters. The device could be dynamically refocused to a shorter distance to address a closer subject (or a subject moving toward the device), but this adds to system complexity. FIGS. 1 (c) and (d) show similar plots to those of (a) and (b), but for devices having a 0.3 meter diameter aperture. The focused device is configured to place the maximum intensity plane at a distance of 10 meters from the device. Again the curves are normalized to a maximum peak intensity of 1 W/cm2. The associated total power requirements to transmit the beams shown are 360 W (per W/cm2) for the collimated beam, and 75 W (per W/cm2) for the focused beam. Here, the collimated beam requires slightly less than 5 times as much power, but again, the focused beam is likely to fall below effective power densities at distances of less than 5 meters unless dynamic focusing is used. The collimated systems have greater “depth of field” (defined here as the range of distance over which the beam maintains a usable power density) than the focused systems, but the collimated systems require much more total output power to reach effective power densities at any distance.
This disclosure describes approaches to improve the effective depth of field as defined above, while reducing the total output power required to achieve effective power densities over a broader range of distances. These approaches can be combined or used separately.